Re: [PATCH v2 1/2] kasan: initialize shadow to TAG_INVALID for SW_TAGS

From: Andrey Konovalov <andreyknvl@google.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Dmitry Vyukov <dvyukov@google.com>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Alexander Potapenko <glider@google.com>,
	Marco Elver <elver@google.com>,
	Peter Collingbourne <pcc@google.com>,
	Evgenii Stepanov <eugenis@google.com>,
	Branislav Rankov <Branislav.Rankov@arm.com>,
	Kevin Brodsky <kevin.brodsky@arm.com>,
	Christoph Hellwig <hch@infradead.org>,
	kasan-dev <kasan-dev@googlegroups.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Linux Memory Management List <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 1/2] kasan: initialize shadow to TAG_INVALID for SW_TAGS
Date: Fri, 5 Mar 2021 16:46:15 +0100	[thread overview]
Message-ID: <CAAeHK+xaPBNB+VpXcj_Xdk0qg-FgDe9i1m4mEY1-ChxQND_8kA@mail.gmail.com> (raw)
In-Reply-To: <c8e93571c18b3528aac5eb33ade213bf133d10ad.1613692950.git.andreyknvl@google.com>

On Fri, Feb 19, 2021 at 1:22 AM Andrey Konovalov <andreyknvl@google.com> wrote:
>
> Currently, KASAN_SW_TAGS uses 0xFF as the default tag value for
> unallocated memory. The underlying idea is that since that memory
> hasn't been allocated yet, it's only supposed to be dereferenced
> through a pointer with the native 0xFF tag.
>
> While this is a good idea in terms on consistency, practically it
> doesn't bring any benefit. Since the 0xFF pointer tag is a match-all
> tag, it doesn't matter what tag the accessed memory has. No accesses
> through 0xFF-tagged pointers are considered buggy by KASAN.
>
> This patch changes the default tag value for unallocated memory to 0xFE,
> which is the tag KASAN uses for inaccessible memory. This doesn't affect
> accesses through 0xFF-tagged pointer to this memory, but this allows
> KASAN to detect wild and large out-of-bounds invalid memory accesses
> through otherwise-tagged pointers.
>
> This is a prepatory patch for the next one, which changes the tag-based
> KASAN modes to not poison the boot memory.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
>  include/linux/kasan.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index 14f72ec96492..44c147dae7e3 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -30,7 +30,8 @@ struct kunit_kasan_expectation {
>  /* Software KASAN implementations use shadow memory. */
>
>  #ifdef CONFIG_KASAN_SW_TAGS
> -#define KASAN_SHADOW_INIT 0xFF
> +/* This matches KASAN_TAG_INVALID. */
> +#define KASAN_SHADOW_INIT 0xFE
>  #else
>  #define KASAN_SHADOW_INIT 0
>  #endif
> --
> 2.30.0.617.g56c4b15f3c-goog
>

Hi Andrew,

Could you pick up this series into mm?

The discussion on v1 of this series was hijacked discussing an unrelated issue.

Thanks!